1. Field of the Invention
The present invention relates to a radiation imaging apparatus and a control method for the same.
2. Description of the Related Art
A radioactive ray (for example X-ray) tube is configured to use a thermal electron source. An X-ray tube emits thermal electrons from a filament which is heated to high temperature and accelerates an electron beam to high energy via a Wehnet electrode, an extraction electrode, an acceleration electrode and a lens electrode. After forming the electron beam into a desired shape, X-rays are generated by irradiation of a metallic X-ray target with the beam.
In recent years, the thermo electron source has been replaced by the development of a cold-cathode electron source. Cold-cathode electron sources have been widely researched as application of flat panel displays (FPD). A representative example of a cold-cathode electron source is a Spindt-type electron source in which electrons are extracted by application of a high electrical field to a tip of a needle measuring tens of nanometer (nm). A surface conduction electron source is known. In a surface conduction electron source, an electron emitter formed from a carbon nanotube (CNT) or from a minute nm-order structure is formed on a surface of a glass substrate and electrons are emitted from the electron emitter.
As application of these electron sources, a technique has been discussed which generates X-rays by forming a single electron beam using a Spindt-type electron source or a CNT electron source as discussed in Japanese Patent Application Laid-Open No. 9-180894 and Japanese Patent Application Laid-Open No. 2004-329784. Further, Japanese Patent Application Laid-Open No. 8-264139 and J. Zhang “Stationary scanning X-ray source based on carbon nanotube field emitters” Applied Physics Letters 86, 184104 (2005), discuss a technique for generating X-rays by irradiation of an X-ray target using an electron beam from a multi electron source which uses a plurality of cold-cathode electron sources. Japanese Patent Application Laid-Open No. 2007-265981, which corresponds to patent application publication US 2009/0316860 and is assigned to the assignee of the present application, discusses a technique of using a multi X-ray source to generate a multi X-ray beam which has excellent properties and avoids mutual interference.
A radiation imaging apparatus has various applications. For example, one of those applications is a mammographic apparatus configured to execute mammography to acquire a radiographic image of a mammary gland, a tumor, or calcification. During mammography, a patient's breast is positioned in a special platform and compressed with a paddle made of clear plastic or the like. Breast compression is necessary, among other reasons, to even out the breast thickness so that all of the breast tissue can be evenly visualized, to spread the tissue so that small abnormalities are less likely to be obscured by tissue, to allow the use of lower X-ray dosage, to hold the breast still while imaging, to reduce X-ray scatter. Notwithstanding breast compression, since a mammographic apparatus provides a mammogram by projection, the mammary gland image sometimes may become blurred due to superimposition of the mammary gland with calcification or a tumor. Statistical data shows that approximately half of the cases in which mammogram re-scanning was required were caused by this type of superimposition with mammary gland tissue. In an attempt to address the superimposition problem, Japanese Patent Application Laid-Open No. 2007-216022 discusses a technique for generating a three-dimensional image in a suspect area of clinical concern by driving an X-ray source at a position along a direction of compression, acquiring a plurality of X-ray images from a group of exposures and using tomosynthesis to reconstruct the images.
Generally, a mammographic apparatus executes bidirectional imaging processing using a craniocaudal projection (CC projection) and a mediolateral oblique projection (MLO projection). With these techniques, images of the breast are captured from a direction along a compression direction. It is, however, sometimes desirable to vary the incident direction of X-ray radiation according to a calcification distribution profile or a tumor profile.
However, a suitable incident direction of X-ray cannot be known in advance. When executing radioscopic imaging other than mammography, a suitable imaging direction is determined and then high-resolution and high-dose still images are captured. When imaging breasts, since a mammographic X-ray source is conventionally positioned slightly higher than the head of a patient, a patient may experience discomfort during displacement of the X-ray source.
In addition, since the X-ray source executes imaging in a stationary state, when the X-ray source is displaced, the apparatus itself vibrates and residual vibration during imaging processing may adversely affect image definition. Moreover, time required for imaging will be increased is a waiting period is taken until such vibration dissipates. Further, a patient will experience considerable discomfort since the breast remains compressed during this time.